This invention relates to a molding die for a molded article made of synthetic plastic and more specifically, to a molding die with a demolding core.
As an example, Japanese Laid-Open Patent Publication No. H10-71634 discloses a conventional molding die relating to a similar technique. As shown in FIGS. 21(a) and 21(b), this molding die includes a movable die 52 by which a bumper 51 with an undercut 50 is formed, and a demolding core 53 that moves relative to the movable die 52 to demold the bumper 51 from the movable die 52. As shown in FIG. 21(b), in response to movement of the demolding core 53, the bumper 51 deforms elastically such that the undercut 50 moves away from the movable die 52, thereby allowing detachment of the bumper 51 from the movable die 52.
If the bumper 51 is one that is to be attached to a vehicle, the bumper 51 generally has rigidity higher at its lower edge part than at its upper edge part, as shown in a partial view of FIG. 22. This is because the bumper merely has an abutment section 51a at its upper edge part that is to abut against right and left fenders or a front grille to be coupled thereto, whereas it has a flange section 51b at its lower edge part that is to extend to face lower part of a vehicle body.
If the bumper 51 is formed by a molding die such as that described in Japanese Laid-Open Patent Publication No. H10-71634 and the molded bumper 51 is to be detached from a movable die, the demolding core 53 is moved in the direction of arrow P to spread the undercut 50 outward (to the right of FIG. 23), as shown in FIG. 23. Lower part of the bumper 51 has high rigidity. Thus, the lower part of the bumper 51 deforms by an amount smaller than the deformation amount of upper part of the bumper 51. This enables the bumper 51 to move in a manner shown by arrow Q of FIG. 23. Specifically, the bumper 51 moves outward in the movement direction of the demolding core 53 and at the same time, it may also move in a direction perpendicular to the movement direction of the demolding core 53. In this case, friction may be caused between a surface of the undercut 50 (surface of a hatched area of FIG. 23) and a corner of the demolding core 53, thereby making a scratch on this surface. Hence, if the bumper 51 is attached to a vehicle and the undercut 50 forms part of the interior of a wheel arch of the vehicle, a problem may be caused that the scratch is visible from outside the vehicle.
Japanese Laid-Open Patent Publication No. H07-52733 discloses a structure where multiple ribs are formed on the rear surface of an undercut of a bumper with the intention of enhancing the rigidity of the undercut of the bumper. However, these ribs increase the weight of the bumper.